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Publication numberUS5268048 A
Publication typeGrant
Application numberUS 07/988,638
Publication dateDec 7, 1993
Filing dateDec 10, 1992
Priority dateDec 10, 1992
Fee statusLapsed
Publication number07988638, 988638, US 5268048 A, US 5268048A, US-A-5268048, US5268048 A, US5268048A
InventorsJacques Leibovitz, Hilmar W. Spieth, Peter F. Dawson, Voddarahalli K. Nagesh
Original AssigneeHewlett-Packard Company
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Reworkable die attachment
US 5268048 A
Abstract
An integrated circuit is reworkably attached to a circuit board in a manner that forms a compliant bond which is stable under conditions of high thermal stress and thermal coefficient of expansion mismatch. A thermoplastic adhesive having a melting temperature higher than integrated circuit operating temperature is coated on the integrated circuit and dried. The adhesive is then cured. The coated integrated circuit is bonded to the circuit board with a thermosetting epoxy having a low curing temperature, such that curing the adhesive does not damage the circuit board. The integrated circuit is readily removed from the circuit board without damaging the board by heating the integrated circuit to soften the bond between the integrated circuit and the circuit board at the thermoplastic adhesive interface. An alternate embodiment of the invention provides a copper plate interface that is soldered to the circuit board, and to which an integrated circuit is permanently bonded. Reworkability is achieved by flowing the solder bond and removing the copper plate/integrated circuit assembly.
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Claims(2)
We claim:
1. A method for forming a releasable, compliant bond between a component and a substrate, comprising the steps of:
coating a bond surface of said component with a thermoplastic adhesive;
drying said thermoplastic adhesive;
curing said thermoplastic adhesive;
adhering said component to said substrate by coating at least one of said component bond surface and a substrate bond surface with a thermosetting adhesive and, thereafter, bringing said component bond surface and said substrate bond surface together; and
curing said thermosetting adhesive.
2. The method of claim 1, further comprising the step of:
releasing said component from said substrate by applying sufficient heat to said component to soften said thermoplastic adhesive rapidly; and
detaching said component from said substrate before damaging said substrate.
Description
BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to electronic assemblies. More particularly, the present invention relates to reworkably attaching integrated circuits and related devices to a substrate.

2. Description of the Prior Art

Semiconductor technology has advanced to the point where a designer has many options to choose from when mounting an integrated circuit die as part of an electronic assembly. For example, it is common practice to mount integrated circuit die directly to various substrates, such as packaging, circuit boards, flexible films, etc.

Circuit boards are presently the predominant means of forming electronic assemblies with integrated circuits and related components. One advantage of a circuit board is that, in addition to providing a substrate to which the integrated circuit may be mechanically attached, for example by an adhesive, circuit boards also provide electrical interconnection between the terminals of the integrated circuit and other electronic components in the electronic assembly. Various techniques are known for making electrical connections between the terminals of the integrated circuit and those of the circuit board, e.g. ball bonding, bumping, wire bonding, etc.

It is desirable, and often necessary, to remove defective or obsolete integrated circuits from such electronic assemblies, such that the assemblies may be reworked. In this way, an expensive circuit board containing many integrated circuits and other components may be repaired and reused, rather than being discarded. It is also desirable to remove integrated circuits from circuit boards for salvage purposes. For example, it is often useful to remove an expensive microprocessor from a circuit board for use elsewhere, or where the board is irreparably damaged.

Most adhesives that are commonly used to attach a die to a circuit board form a permanent bond between the integrated circuit and the circuit board. When these adhesives cure, an irreversible polymerization occurs. Once bonded to a circuit board in this way, a die may not be removed from the circuit board without destroying the die and damaging the board, along with other electronic components mounted on the board.

It is known that thermoplastic adhesives provide a bonding material that may be softened with heat and thereby reworked. However, the selection of softening and curing temperatures for such adhesives is quite limited. Thermoplastic adhesives that may be cured and softened at temperatures low enough to avoid damaging common circuit boards, such as are made of FR4 material having a low glass transition temperature ("Tg"), and neighboring electronic components during attachment or rework do not maintain a satisfactory bond under normal circuit operating conditions. Thus, a thermoplastic bond will fail when the bond is subjected to significant thermal stress, such as is typically found in a powered integrated circuit. Additionally, the likelihood of bond failure due to thermal stress is exacerbated by the temperature coefficient of expansion (TCE) mismatch between the silicon integrated circuit, about 2.3 cm/cm- C., and the circuit board material, about 17 cm/cm- C.

Thus, adhesives that provide a reworkable, stable bond for attaching an integrated circuit die to a common circuit board are not known. A process for reworkably attaching a die to a circuit board would provide more efficient electronic module production by eliminating the waste of otherwise repairable circuit boards and other components.

SUMMARY OF THE INVENTION

The present invention provides a strong, highly compliant, reworkable bond for attaching an integrated circuit die to a common circuit board. The bond provided by the present invention is stable under conditions of high thermal stress, and is especially suited for materials having differing thermal properties, for example materials of low Tg and/or where a high thermal coefficient of expansion mismatch is experienced.

In the preferred embodiment of the invention, a thermoplastic adhesive is applied to an integrated circuit. The adhesive is then dried and cured. After coating with thermoplastic adhesive, the integrated circuit is adhered to a printed circuit board with a high compliance, thermosetting epoxy adhesive having a low curing temperature.

The thermoplastic adhesive is selected from those adhesives having the lowest possible curing temperature, but a sufficiently high curing temperature, such that a stable bond is maintained at temperatures experienced when an integrated circuit is powered and operating. The thermosetting epoxy is selected for its low curing temperature, such that curing of the adhesive does not damage the low Tg printed circuit board.

During rework, intense heat is applied to the integrated circuit, and first flows to the thermoplastic adhesive, thereby softening the adhesive. The softening of the adhesive bond allows the detachment of the integrated circuit from the circuit board, which immediately stops heat flow through the integrated circuit to the circuit board, and prevents damage to the printed circuit board.

An alternate embodiment of the invention provides a copper plate to which an integrated circuit is bonded with a thermosetting adhesive. The copper plate/integrated circuit assembly is then soldered to a printed circuit board by any of the known methods, including surface mount technology.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective representation of an integrated circuit/circuit board bond formed according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is best understood by referring to the Drawings in connection with review of this Description. The present invention provides a highly compliant, stable, reworkable bond, exhibiting excellent mediation of thermal properties between silicon and various substrate materials, including materials having a low Tg and/or large coefficient of expansion mismatch.

FIG. 1 is a schematic perspective view of an integrated circuit 10 that has been bonded to a printed circuit board 14 in accordance with the present invention. The present invention provides a composite bond 16/18 in which a layer of thermosetting adhesive 18 is included to provide high compliance under conditions of thermal stress and thermal coefficient of expansion mismatch, and more importantly, which cures at a temperature low enough so as not to damage the printed circuit board; and in which a layer of thermoplastic adhesive 16 is included to provide a reworkable bond, such that the integrated circuit 10 may be readily removed from the printed circuit 14 by the application of heat.

The present invention as described herein is directed to bonding a component, such as a silicon integrated circuit to a printed circuit board. It will be appreciated by those skilled in the art that the present invention is readily applicable to other component compositions, for example GaAs, etc. integrated circuits, modules, etc., as well as leadless chip carriers, and various circuit board compositions, for example ceramics, silicon, glass, glass/ceramic, FR4, etc.

In one embodiment of the invention, a thermoplastic adhesive, such as Staystik 181™ manufactured by Staystik, Inc. of Santa Ana, Calif., is applied to a bond surface 11 of an integrated circuit 10. The adhesive is dried and cured. Curing typically is accomplished by heating the adhesive to 220 C. for about 30 minutes. Such adhesive may not be used with printed circuit boards having a low Tg, such as FR4, because the amount of heat required to cure the adhesive would ruin the board.

After the thermoplastic adhesive has cured, the coated surface of the integrated circuit 10 is bonded to a bond surface 15 of the printed circuit board 14 with a thermosetting adhesive having a low curing temperature, such as Ablebond™ 84-1A, manufactured by Ablestik Laboratories of Rancho Dominguez, Calif. The low curing temperature of the thermosetting adhesive is consistent with the low Tg of common circuit board materials and any thermosetting adhesive exhibiting this quality may be substituted for the exemplary adhesive without departing from the spirit and scope of the present invention.

The composite bond thus formed between the integrated circuit and the circuit board may be readily reworked, e.g. to replace a defective integrated circuit, or to salvage a valuable integrated circuit for use elsewhere. Such reworkability is particularly important in most modern applications where a printed circuit board typically contains dozens of valuable components. The ability to remove and replace a defective component, as provided by the present invention, assures that functioning devices and otherwise serviceable circuit boards are not discarded. Thus, product manufacturing costs are significantly reduced.

For rework, the integrated circuit is heated, thereby softening the thermoplastic adhesive (which typically softens at about 180 C. after 1-5 minutes of heating). The softening of the adhesive bond allows the integrated circuit to be removed from the printed circuit board. Removing the integrated circuit immediately stops the flow of heat to the circuit board and thereby prevents damage to the printed circuit board. That is, the adhesive bond begins to soften before sufficient heat is transferred to the printed circuit board to damage the board. It is therefore beneficial to remove the integrated circuit from the printed circuit board quickly and thereby limit circuit board exposure to heat.

After a defective integrated circuit is removed from a printed circuit board, a replacement integrated circuit may be attached to the printed circuit by the same technique. That is, a thermoplastic adhesive is applied to the replacement integrated circuit, the adhesive is dried and cured, and the coated surface of the integrated circuit is bonded to the printed circuit board with a thermosetting adhesive. As the original thermosetting layer remains on the surface of the printed circuit, it is necessary to bond the replacement integrated circuit to the printed circuit with an additional thermosetting adhesive layer.

An alternate embodiment of the invention provides an interface substrate, such as a copper plate to which the integrated circuit is bonded with a thermosetting adhesive which provides mediation of TCE mismatch. The assembly thus formed is bonded to the circuit board with solder by any of the known techniques currently used in industry.

Thus, two structural bond configurations are provided by the present invention: circuit board/thermosetting adhesive/thermoplastic adhesive/Si; and circuit board/solder/Cu/thermosetting adhesive/Si.

Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the claims included below.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US4012832 *Mar 12, 1976Mar 22, 1977Sperry Rand CorporationMethod for non-destructive removal of semiconductor devices
US5137836 *May 23, 1991Aug 11, 1992Atmel CorporationMethod of manufacturing a repairable multi-chip module
US5220724 *Sep 9, 1991Jun 22, 1993Robert Bosch GmbhMethod of securing surface-mounted devices to a substrate
EP0051165A1 *Oct 9, 1981May 12, 1982BURROUGHS CORPORATION (a Michigan corporation)Repairable IC package with thermoplastic chip attach
JPS5944834A * Title not available
Non-Patent Citations
Reference
1Dunkel, W. E. IBM Technical Disclosure Bulletin "Replaceable Chip to Heat Sink Connection on Circuit Boards" vol. 14, No. 11 Apr. 1972.
2 *Dunkel, W. E. IBM Technical Disclosure Bulletin Replaceable Chip to Heat Sink Connection on Circuit Boards vol. 14, No. 11 Apr. 1972.
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US5601675 *Dec 6, 1994Feb 11, 1997International Business Machines CorporationReworkable electronic apparatus having a fusible layer for adhesively attached components, and method therefor
US5607538 *Sep 7, 1995Mar 4, 1997Ford Motor CompanyMethod of manufacturing a circuit assembly
US5739586 *Aug 30, 1996Apr 14, 1998Scientific-Atlanta, Inc.Heat sink assembly including a printed wiring board and a metal case
US5749988 *Sep 21, 1994May 12, 1998Leibovitz; JacquesReworkable die attachment to heat spreader
US5757073 *Dec 13, 1996May 26, 1998International Business Machines CorporationHeatsink and package structure for wirebond chip rework and replacement
US5769989 *Sep 19, 1995Jun 23, 1998International Business Machines CorporationMethod and system for reworkable direct chip attach (DCA) structure with thermal enhancement
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US5953210 *Jul 8, 1997Sep 14, 1999Hughes Electronics CorporationReworkable circuit board assembly including a reworkable flip chip
US6029730 *Dec 30, 1997Feb 29, 2000Intel CorporationHot shear apparatus and method for removing a semiconductor chip from an existing package
US6134776 *Feb 5, 1998Oct 24, 2000International Business Machines CorporationHeatsink and package structure for wirebond chip rework and replacement
US6238223 *Aug 20, 1997May 29, 2001Micro Technology, Inc.Method of depositing a thermoplastic polymer in semiconductor fabrication
US6319846 *Jan 5, 2001Nov 20, 2001Taiwan Semiconductor Manufacturing Company, LtdMethod for removing solder bodies from a semiconductor wafer
US6380322Jun 17, 1999Apr 30, 2002Georgia Tech Research CorporationReworkable high temperature adhesives
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US7107661 *Dec 16, 2002Sep 19, 2006Denso CorporationMethod of recycling printed circuit board
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US20040055152 *Sep 25, 2003Mar 25, 2004James FraivilligBonding of a multi-layer circuit to a heat sink
US20040111854 *Dec 16, 2002Jun 17, 2004Rikiya KamimuraMethod and appratus of recycling printed circuit board
US20090111222 *Oct 23, 2008Apr 30, 2009Kazutaka YoshidaSemiconductor chip mounting method, semiconductor mounting wiring board producing method and semiconductor mounting wiring board
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Legal Events
DateCodeEventDescription
Feb 11, 1993ASAssignment
Owner name: HEWLETT-PACKARD COMPANY, CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNORS:LEIBOVITZ, JACQUES;SPIETH, HILMAR W.;DAWSON, PETER F.;AND OTHERS;REEL/FRAME:006409/0669;SIGNING DATES FROM 19921109 TO 19921210
Jul 15, 1997REMIMaintenance fee reminder mailed
Dec 7, 1997LAPSLapse for failure to pay maintenance fees
Feb 17, 1998FPExpired due to failure to pay maintenance fee
Effective date: 19971210